The present invention relates to accelerometers, and in particular to structures for mounting the same, whereby external stress sources are isolated from active accelerometer components.
Accelerometers generally measure acceleration forces applied to a body. Accelerometers are typically mounted directly onto a surface of the accelerated body. Such direct mounting ensures the immediate detection of even subtle forces exerted on the body. The directly mounted accelerometer is, however, also exposed to various extraneous shock, vibration and thermal stresses experienced by the accelerated body. The accelerometer measures the forces induced by such external stresses in combination with the applied acceleration forces and renders confused and inaccurate acceleration measurements. Generally, isolation mechanisms between the accelerometer and the accelerated body are typically integrated into the accelerometer housing to protect the accelerometer from forces induced by stresses within the accelerated body.
Additionally, sensitive accelerometers can suffer from error sources caused by subtle forces induced by stresses internal to the accelerometer but external to the acceleration sensing mechanism. In monolithic micro-machined accelerometers having vibrating beam force detectors suspended between a movable proof mass and an accelerometer frame, such forces are caused by, for example, mounting stresses between a silicon cover plate and the sensor frame or other assembly stresses. Other such stresses include, for example, thermal stresses resulting from a mismatch of thermal expansion coefficients between materials within the sensor. External thermal stresses may be induced by the typical mechanical coupling of the sensor frame to the silicon cover plate and by the mechanical coupling of the silicon cover plate to a ceramic or metal mounting plate. Since the cover and mounting plates are typically fabricated of materials different from the sensor frame, they usually have substantially different coefficients of thermal expansion. When operated at elevated temperatures, the mismatch in thermal expansion coefficients generally causes undesirable stresses which induce distortion and strain in the sensor frame.
Bias performance and stability of monolithic silicon-based accelerometers is based on proof mass sizing, commonly referred to as pendulousity, and on the degree of stress isolation in the mechanical die stack. Monolithic micro-machined vibrating beam accelerometers are typically targeted for small size which limits the proof mass size and generally requires special care in providing isolation from external stresses. Historically, the accelerometer frame is suspended from a second outer frame by flexures that permit the accelerometer frame to move relative to the outer frame, as shown and described in allowed U.S. patent application Ser. No. 08/735,299, now U.S. Pat. No. 5,948,981 to Woodruff entitled, VIBRATING BEAM ACCELEROMETER, issued Sep. 7, 1999. Such isolation structure designs as have been possible using a potassium hydroxide (KOH) etching solution in a bulk process to cost effectively fabricate monolithic micro-machined vibrating beam accelerometers effectively minimize the distortion of the accelerometer frame and decrease the effects of the thermal coefficient mismatch. However, the orientation of the natural etch planes in silicon at 57.4 degrees from vertical using a KOH etching solution requires relatively large amounts of physical space, thus limiting both the pendulousity, i.e., possible proof mass size, and the possible isolation structure designs and requiring major compromises and trade-offs in proof mass sizing and isolation structure design in very small applications.
In prior art devices, the flexures that suspend the accelerometer frame from the second outer frame are commonly compliant beam or spring isolators. These compliant beam or spring isolators are used to reduce the stresses caused by mounting displacements to a small value. These isolators obey a simple spring equation, given by:
Force (F)=spring constant(k)*displacement(d). 
Thus, for a given mounting displacement, the force applied to the sensor is reduced through the isolator spring constant, which is designed to be as low as possible. The resulting strain in the sensor is thus reduced through the spring constant of the isolator.
A typical example of such compliant beam or spring isolators is found in the twin beam suspension system illustrated in FIG. 1. The accelerometer illustrated in FIG. 1 has a conventional isolation structure formed of compliant beam or spring isolators embodied as flexures. In FIG. 1 the accelerometer 10 is a miniature structure fabricated from a substrate 12 of semiconductor material by conventional micromachining techniques. The substrate 12 is formed of a monocrystalline silicon material. The silicon substrate 12 often includes an upper silicon or active layer 14 that is electrically isolated from an underlying substrate 16 by an insulating layer 18 or an insulating layer is applied to active layer 14, as shown and described in incorporated U.S. Pat. No. 5,948,981. The insulating layer 18 is may be a thin layer, e.g., about 0.1 to 10.0 micrometers, of oxide, such as silicon oxide. The silicon substrate 12 is usually formed by oxidizing active layer 14 and underlying substrate 16, and adhering the two layers together. A portion of active layer 14 may be removed to bring the layer 14 to the desired thickness. The silicon oxide layer 18 retains its insulating properties over a wide temperature range to ensure effective mechanical resonator performance at high operating temperatures on the order of 100 degrees Celsius. In addition, the insulating layer 18 inhibits undesirable etching of the active layer 14 during etching.
The accelerometer 10 includes an acceleration sensor mechanism 20 having one or more flexures 22 pliantly suspending a proof mass 24 from a sensor frame or plate 26 for movement of the proof mass 24 along an input axis I normal to the proof mass 24. The flexures 22 are preferably etched near or at the center of the underlying substrate 16, i.e., substantially centered between the opposing upper and lower surfaces of the underlying substrate 16. Optionally, the flexures 22 are formed by anistropically etching in a suitable etchant, such as potassium hydroxide (KOH). The flexures 22 define a hinge axis H about which the proof mass 24 moves in response to an applied force, such as the acceleration of the accelerated body, for example, a vehicle, aircraft or other moving body having the accelerometer 10 mounted thereon. The sensor mechanism 20 includes a pair of mechanical resonators 28 formed from the active silicon layer 14 and coupled between the proof mass 24 and the sensor plate 26 for measuring forces applied to the proof mass 24. An oscillator circuit (not shown) drives the mechanical resonators 28 at their resonance frequency. In response to an applied force, the proof mass 24 rotates about the hinge axis H, causing axial forces, either compressive or tensile, to be applied to the mechanical resonators 28. The axial forces change the frequency of vibration of the mechanical resonators 28, and the magnitude of this change serves as a measure of the applied force or acceleration.
External stresses and strains may be induced in the sensitive acceleration sensor mechanism 20 by, for example, the typical mechanical coupling of the accelerometer sensor plate 26 to a silicon cover plate 30. The silicon cover plate 30 is in turn typically connected to a ceramic or metal mounting plate 32. Since the mounting 32 and cover plates 30 are fabricated from different materials, they will usually have substantially different coefficients of thermal expansion when cooled or heated during operation. This mismatch in thermal coefficients may cause undesirable stresses and strains at the interface of the inner and cover plates, causing a slight distortion of the sensor plate 26. Other stresses and strains induced in the sensitive acceleration sensing mechanism 10 include, for example, external sources of shock and vibration experienced by the accelerated body and the accelerometer 10. Many methods of isolating the sensor plate 26 from such undesirable stresses and strains are known to those of ordinary skill in the relevant arts. For example, suspending the sensor plate 26 from a second outer or external frame 34 by flexures 36 formed by overlapping slots 38 and 40 through the substrate 12. The sensor plate 26 is thus able to move relative to the outer frame 34, as shown and described in U.S. Pat. No. 5,948,981, which is assigned to the assignee of the present application and is incorporated herein by reference. Such isolation minimizes the distortion of the sensor plate 26, and thereby decreases the effects of thermal mismatching on the mechanical resonators 28.
The prior isolation approach illustrated in FIG. 1 suffers limitations. While the stress and resulting strain imposed on the sensitive acceleration sensor mechanism 20 may be substantially reduced, they are never completely eliminated. Much effort is generally required to find a xe2x80x9cbest support locationxe2x80x9d or xe2x80x9cbest configurationxe2x80x9d such that the impact to the sensor mechanism 20 is minimized. Also, the compliance of the isolator, flexures 36 in the device of FIG. 1, introduces a lower frequency resonance condition into the accelerometer system. This lower frequency resonance condition is problematic in most practical applications because practical applications generally require resonances that are as high a frequency as possible to ensure accurate acceleration measurements.
In any practical suspension system design the requirement to isolate the sensor mechanism from external stresses is in opposition with the requirement to ensure accurate acceleration measurements. In practical sensors the need to keep resonant frequencies high limits the compliance of the isolation beams or flexures, and therefore, limits the isolation that can be obtained with normal compliant spring isolators.
The present invention overcomes the accelerometer and proof mass sizing constraints of the prior art by providing a method and apparatus which provide the acceleration sensor device isolation from mounting stress and shock, vibration and thermal stresses experienced by the accelerometer. The method and apparatus of the invention also achieve a rigid isolation structure having a high resonance frequency. The acceleration sensor device of the invention nulls mounting point displacements to zero at sensor support points, and accomplishes this nullification of mounting point displacements with a structure that may be very rigid.
According to one aspect of the invention, a net zero isolator is provided having an elongated displacement reaction member; and first and second counter rotation members arranged crosswise to the displacement reaction member at either end thereof, each of the first and second counter rotation members including a mounting portion spaced apart from an isolated portion on respective first and second sides of the elongated displacement reaction member. The elongated displacement reaction member and the first and second counter rotation members are structured such that a displacement of the mounting portions along the displacement reaction member is balanced by a displacement of the isolated pads. The net zero isolator acts through each of the first and second counter rotation members to generate deflections of the first and second counter rotation members that cancel a linear displacement of the mounting portions at the isolated portions.
According to another aspect of the invention, a two-axis suspension system is provided having a pair of the net zero isolators of the invention combined to provide a stable, two-axis suspension system or sensor platform.
According to still another aspect of the invention, an accelerometer is provided having the acceleration sensor device in combination with a plurality of the net zero isolators of the invention embodied as a two-axis suspension system, whereby the acceleration sensor device is isolated from mounting stress and shock, vibration and thermal stresses experienced by the accelerometer.